An analytical approach in calculating electron field emission current from a silicon subsurface layer is presented. By employing a nontriangular quantum well model for the silicon subsurface layer, wave functions of electrons in the silicon subsurface layer are given on the basis of generalized Airy functions. After tunneling through a vacuum barrier, the electrons from the silicon subsurface layer are-represented by outgoing waves at infinity. Such a boundary condition results in a complex subband-state energy, which simultaneously provides a subband energy level and: a tunneling probability per unit time. A negative differential resistance characteristic is shown, being consistent with the experimental result.